


Fever Pitch

by dunked_delirious



Category: Undertale (Video Game)
Genre: Aftercare, Alternate Universe - Underfell (Undertale), Big Sans, Biting, Crying, Dirty Talk, Dom Sans, Dom/sub, Ecto-Penis (Undertale), Ecto-Tongue (Undertale), Established Friends With Benefits, F/M, Fluff and Smut, M/M, Mating Cycles/In Heat, Multiple Orgasms, Other, Overstimulation, Plot What Plot/Porn Without Plot, Reader Is Not Chara, Reader Is Not Frisk, Reader has a vagina, Reader has no defined gender, Rough Sex, Size Difference, Vaginal Sex, author has one brain cell, slight Breeding Kink
Language: English
Status: Completed
Published: 2019-07-20
Updated: 2019-07-20
Packaged: 2020-07-08 22:09:07
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,622
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/19876858
Author URL: https://archiveofourown.org/users/dunked_delirious/pseuds/dunked_delirious
Summary: In which Red suffers the seasonal wrath of monster biology, and reader embraces their hidden talents as a heat sink.Reader has a vagina, no gendered language is used.





	Fever Pitch

**Author's Note:**

> One day I'll stop returning to scrapped projects, and actually work on writing the shit I want to write. That day is not today. Until then, enjoy this product of procrastination.
> 
> i've already taken a dive in the deep end, so here's an obligatory heatfic. originally an exercise in writing kinks that aren't my kinks ~~but that i may or may not have acquired in the process~~. Please don't approach this with high expectations.
> 
> also, i'm not even going to try hiding the fact that a little under 1/4 of this is ~~pillow banter~~ pillow talk. heat or no heat, i don't see Red letting his partner off without some much-needed TLC.

In the six months that you’d known him, Sans had cancelled on you twice.  
  
  
The first time, he’d gotten into an MVA en route to a fuckdate. Someone had rammed into his side at an intersection, knocking him off his motorbike and sending him tumbling down the roadway. He’d called you from the hospital, sounding way too chipper for someone who’d cracked half his ribs and broken his arm in two places while he told you, with no shortage of pride, of the shortcut that had saved him from flying into oncoming traffic, and instead sent him crashing through the window of a nearby flower shop. Allegedly, he’d asked the panicked personnel to pack him a bouquet of roses before being picked up by paramedics.  
  
The second time he’d texted you, saying something had happened to his brother. You knew him well enough to know not to press him for the details, but what you were able to glean from the sparse account he gave you later amounted to the relapse of some kind of panic disorder. After that initial text, Sans had gone off radar for a week, and you’d spent the whole time on edge, fearing the worst. You’d learned afterwards that he’d been staying with his brother around the clock. That was all you knew, and Sans was getting visibly upset talking about it, so you left it at that. You’d already taken away what you could from the situation, and it was a cemented belief that if Sans ever bailed out on you, it was for a damn good reason.  
  
That’s why you were worried this third time, staring dumbly at the text he’d sent you yesterday, at the reasonable time of three past five in the morning. _“can’t make it tday, sth came up. nothng big ok i’ll mke iut up to u later”._ While his reassurances provided a modicum of relief, the vagueness of the statement made you uneasy, as did the time of the text. You knew from ample experience that getting Sans up before 10 a.m. was a feat feasible only for his brother. It didn’t help that the texts you’d sent him after—one in response to his and another to wish him good night, then a third one to check on him in the morning—had all gone unanswered. You’d tried calling twice, at different times of day, only for both calls to flatline after a single tune. You were starting to feel intrusive. What if he just wanted to be left alone?  
  
There’s one more way to reach him that you haven’t yet tried, and you make a deal with yourself to make it your final attempt as you dial the number to his homephone. The landline was a last resort you rarely turned to—mostly because Sans, if he was even at home, was rarely willing to expend the effort of answering it. That in turn meant your call was much more likely to be picked up by Boss, which, as you'd figured from the tone of his voice, was an equally unpleasant experience for you both.  
  
Still, desperate times call for desperate measures—and if you’re lucky, Papyrus might just shed some light on what the fuck is up with his brother.  
  
  
  
Your call is picked up after two tones, a familiar scratchy voice sounding from the receiver. “Whatever you are selling, I don’t want it.”  
  
“Papyrus, wait, it’s me!”  
  
He remembers voices better than names, and you can tell he recognizes yours from the way his tone grows marginally less pissy. “What do you need?”  
  
“I, uh, wanted to ask you if Re— if Sans is alright? I heard something went down a few days ago, and he’s been on radio silence ever since—”  
  
“He hasn’t told you?” Papyrus cuts you off sharply. He sounds confused, not distressed, which at least assuages your worries for the worst.  
  
“Uh, pretty sure he hasn’t? He just said something came up.”  
  
There’s a drawn-out sigh from the other end of the line. You have been its recipient enough times in person that you clearly envision the way he’s pinching the bridge of his nasal bone. Then Papyrus states, as if it’s supposed to answer all of your questions, “It is spring.”  
  
“Yes I’m aware, but what does that have to—”  
  
“Sans is in heat.”  
  
You blink, dumbfounded. “In heat,” you echo. _‘Like animals?’_ is on the tip of your tongue, but you don’t voice it. Might’ve gotten an honest laugh out of Red, but if there’s anyone to take offense to the comparison, that would be Boss.   
  
He seems to take your silence for ignorance, and huffs an irritated sigh. “Colloquially speaking, my brother is physically pained by his cravings to fuck anything that moves. More so than what's normal for him, anyway.”  
  
Yeah, sounds like the kind of heat you know about alright. Even so, that leaves you with even more questions. “So is it, like… mating season? Will Sans be alright? How long will he—”  
  
“I am not here to provide you with a treatise on monster sex, human.” You shut up, and Papyrus pauses, before wryly adding, “In fact, by now I would expect you to be an expert on the subject.”  
  
You wince. There’s not much you can say in your defense for the times you’d failed to account for Sans’s not-so-soundproof walls.  
  
You don’t really get the chance to before Papyrus speaks again, this time with a sigh that sounds resigned more than anything. “He’ll be fine. It is a biological function, not a disease. The problem is we cannot control it any more than you humans do the… bleeding.” He says the last part like it's the title of a horror movie. Putting things in perspective, you don’t blame him.  
  
“And he can’t, uh, lend himself a hand?”  
  
Papyrus makes a sound as though you’d suggested something preposterous. “That would defeat the point of a heat, would it not?”  
  
You suppose it would. You’re starting to feel where this is going.

You chew on your bottom lip as you scramble for a tactful way to phrase it. “Would it help to… have someone with him?”  
  
Another scoff from the other end of the line. “I am here to make sure he drinks and eats. I doubt his heat calls for moral support, human.”  
  
You grit your teeth and fight the urge to groan out loud. By all means, you’re glad you’re learning things, but _stars_ do you wish you weren’t having this conversation with Sans’s brother.

“Not like that. I mean someone he can… be with. Sexually.” You cringe at your own wording. Your cheeks burn. It’s grade school sex-ed all over again.  
  
Papyrus falls silent. You’re half convinced he’d gotten fed up and left without hanging up so that you couldn’t call again, but…  
  
“That could work.” You perk up at the sound of his voice from the receiver. “You are not bonded, so it won’t sate his heat, but it might alleviate it for a while.”  
  
You have no clue what the fuck he’s on about, but he had you at ‘alleviate’.  
  
“Then I’ll come over.”  
  
Might be you imagining things, but you think there’s a hint of relief in Papyrus’s voice when he answers. “If you do, get moving so I can let you in. I have places to be.”  
  
  
  
Halfway out your door, you wonder briefly if you know what you’ve signed up for.  
  


* * *

  
  
The ride to the brothers’ place is a fast one, and you find yourself grateful for that as you bound up the stairs to their apartment door. You’ve barely touched your finger to the doorbell before the door flings open, and you find yourself faced with the familiar physiognomy of the Great and Terrible Papyrus. You must have kept him waiting, because he’s already in his leather jacket and biker boots, and has a duffel bag slung over his shoulder.  
  
“Sans is in his room. I will be gone until morning. Help yourself to the fridge if you are hungry. If you do, do me a favor and force Sans to eat as well.” You open your mouth to respond but he’s already out the door, and you’re left to gape at the dwindling drumbeat of his footsteps as he sprints down the stairwell. You shake your head with a sigh, figuring the lack of a lecture is as warm a welcome as you could hope for as you shut and lock the door behind you, trying to ignore the way your stomach flutters at the thought of what’s to come.  
  
  
The route to Sans’s room is a familiar one, laden with many a memory of wandering hands and filthy promises growled against your ear. Absentmindedly, you ponder if association is to blame for the tingle in your thighs, but the thought dies out the instant you walk through that door.  
  
The first thing that hits you is the smell. It’s hot and tart and musky, and somehow not unpleasant; at the base of it is the scent of _him_ , of spice and motor oil and burning, and the sweetness of it permeates the twilight of the room, all the way from where you’re standing in his doorway, to the wide open window behind the billowing curtains and the monster sitting cross-legged on the bed, watching you with smoldering eyelights.  
  
A hot flush floods your cheeks at the sight of him like this, bare save for a pair of shorts, chest rising with his breaths in heaves that seem almost labored. His cheeks match yours, flushed deep with crimson magic, giving off the same eerie light that glows in his joints and burns bright at the center of his sternum. A more familiar glow emanates from his shorts, makes your blood run hot and your skin tingle.  
  
Your underwear would be done for with or without the additional treat of his voice; deep and gruff and _raw_ in a way you’d never heard before. “why th’ fuck’d ya come?”  
  
There is no malice to his tone, only the hint of breathlessness and something that you can’t quite place. Fatigue? Desperation?  
  
“Well, I didn’t.” You take a few steps forward, sounding braver than you feel. “Not yet, at least. Was hoping you’d help me with that.”  
  
 _Hunger._

Sans’s sockets slip shut as he shakes his head with a chuckle. “ya don’t know what yer askin’ fer, kitten.”  
  
The gravelly edge to his voice bolsters your confidence, and you smirk as you take another determined step forward. “Pretty sure I do. Your brother caught me up on things, and I—”  
  
The room spins, and then the wind is knocked from your lungs as you’re slammed against the padded wall, Sans’s fanged grin mere inches from your face. Even in the dim light of the room, there’s no missing the sheen of sweat that coats his bones. “i don’t think ya get it, sweetheart.” The low drawl of his voice makes you weak in the knees, his breath scalding hot against your cheek. “humans aren’t made ta take monster heats.”  
  
You swallow to get some moisture in your throat. You’re starting to get why they call it a heat; Sans’s body radiates it like a nuclear reactor. You feel yourself breaking a sweat just being in his presence. “Why not?”  
  
“’cause it ain’t a nut-‘n-go kinda deal.” His hand hovers at your hip, and with your nod of permission, he crowds you against the wall. The hard press of his cock is nothing new to you, but never before has it felt this _hot_. “if ya stay, i won’t be done with ya fer hours.” A hard pulse of his hips has you gasping—both at the unexpected friction, and the pure, unfettered _relief_ in his groan when he buries his face in your shoulder. “gonna want ta keep givin’ it to ya ‘til my heat runs out. ‘til ya can’t take no more.”  
  
“That sounds like the opposite of a problem.” Your arms come up around his neck to pull him closer. Through the barrier of your shirt and his sternum, you can feel the otherworldly thrum of his soul, tenfold stronger than you’ve ever felt on the nights spent with your cheek pressed to his chest.   
  
Sans is rutting against you in earnest; each roll of his hips pulls low, desperate noises from his throat. “might hurt ya.”  
  
“You know I like that.”  
  
“might say some weird shit.”  
  
“What kind of weird shit?”  
  
“weird possessive shit.” You give an experimental thrust into his motions, and his grip on you tightens as he grits out a throaty moan. “that i wanna claim ya. that yer mine.” Sans’s voice trails off to a growl. “that i wanna _breed_ ya.”  
  
You shudder. Somehow, that’s not a deterrent.  
  
“You— you can’t actually knock me up like this, can you?” you ask sheepishly. He’s told you multiple times that he can’t, but you wouldn’t be surprised if heats are a different breed.  
  
You feel him chuckle against your skin. “nah, ‘s not how monster sex works.”  
  
You’re almost tempted to ask him how it does work, but the graze of sharp teeth against your neck puts an end to your curiosity.

“Then do it,” you breathe. Your hand finds his and guides it to your zipper. “Claim me. I’m all yours.”  
  
You’re not sure what happens the instant the last word leaves your mouth—and even afterwards, you can bet your ass you never saw him moving. All you know for sure is that your mind couldn’t be farther from the syllogisms of space and time as Sans throws you onto his bed, the wind knocked out of you once again as he looms above you, eyelights red and ravenous.  
  
“damn right ya are.” A rough hand rucks up your shirt, baring you to him, and he growls at the whimper you let out when a clawed finger rolls over your nipple. “my dirty lil’ human, comin’ all the way here offerin’ ta be my heat whore.” Your hands shake as you squirm in his grasp, somehow managing to wrestle out of your shirt while Sans pops the button on your jeans. You barely have the presence of mind to lift your hips when he all but tears your pants off along with your underwear, and you’re struck with the first peal of dread for the night as it hits you in full just how breakable you are, naked at the mercy of a heat-crazed monster.  
  
You cry out as two fingers plunge inside you without warning, mouthing a silent thanks for the retracted claws. Sans’s eyelights are locked on your cunt as he stretches you, a breathy groan escaping him at the wet squelch of his fingers. “fuckin’ hell, an’ i thought i had it bad.” You feel each hard knuckle as he slowly withdraws, giving your clit a playful flick before he splays his hand and watches as your slick clings to his fingers. “yer gorgeous body knows who it belongs ta, don’t it?”  
  
You open your mouth to answer, but all that escapes is a startled cry as Sans shoves your legs apart, hitching them over his shoulders and crawling over you until your knees are pressed to your chest. You feel the hot weight of his cock where it nudges at your entrance, and you arch to try and roll your hips against it, only for Sans to growl and hold you steady, hard fingers pressing into the flesh of your thighs. He’s rattling hard, sweat streaking down his bones as his entire body trembles.  
  
“promise me,” he rasps, his voice a breathless wreck, “promise ta tell me if ya need me t’ stop.”  
  
“I promise,” you whimper, locking your legs around his neck. The wide-eyed, borderline deranged look that earns you is all the warning you get before he rams himself inside you, hilting with a single, brutal thrust.  
  
Your entire body arches up on impact, a cry ripped from your throat as your mind reels under the onslaught of trying to fathom everything you’re feeling. Sans doesn’t give you the time—he pulls back and thrusts again, ruts into you like a beast in heat, and you can swear he feels bigger than before, stretching you to your limit.  
  
“oh fuckin’— _oh,_ bitchin’ _stars—!_ ” Sans’s voice is as close to a whine as you’ve ever heard it, expression wracked with equal parts relief and agony as he plows into you with reckless abandon. His sockets are squeezed shut in a rare breach of normalcy—the Red you know wouldn’t miss a second of you squirming—and you promptly get your answer as for why when he cums inside you with a strangled groan, offering a fleeting reprieve from his ruthless rhythm as his magic coats your walls.  
  
You didn’t expect there to be so much _more_ of it than what you’re used to. There’s too much for you to keep inside, and you can feel the excess running down your thighs and the curve of your ass by the time Sans finally stills. Breathless and light-headed, you can barely focus your eyes enough to parse his predatory grin, and the look he gives you when it dawns on you that he’s still perfectly hard. “ya still with me, sweetheart?” Not quite trusting yourself with words, you settle for nodding, and he chuckles. “good, ‘cause there’s helluva lot more where that _came_ from.”  
  
You get no more than a split second to groan at the pun before you’re unceremoniously flipped onto your hands and knees, and then you're groaning for a completely different reason as Sans slams back inside you. The angle is different like this, the added slick of his release easing the way as you keep taking his monster of a cock, each delicious drag of his ridges pulling broken whimpers from your throat. Your hands fist in his sheets as you cling on for dear life, each brutal thrust jostling you forward on the mattress, and the coarse fabric of his bedspread rubs ruthlessly against your aching nipples.  
  
One clawed hand slides from your hip down to your stomach, pulling you into his thrusts, and his cock hits something inside you that has you seeing stars. “Oh fuck, _Sans—!_ ”  
  
“that’s it, fuck—!” Sans sounds as ruined as you feel, a growl on the tail end of each breath. “fuckin’ scream fer me, _scream_ my name!”  
  
Sharp teeth sink into your shoulder, and that’s what does you in. Your entire body quakes as you scream your release, noises you never knew you were capable of making punched out of you with each unrelenting thrust. You think you feel Sans cum again while you’re shaking through your climax; you can’t be sure. His hands, his cock, his breath on your neck and the weight of his body on yours—everything around you is so _hot_ that you’re burning up with it. There’s no room in your thoughts for anything else but the monster on you, around you, _inside_ you—all you know for certain is how you don’t want him to stop, and you tell him that much, _beg_ him to keep nailing you to his mattress even as your legs ache and overstimulated tears run down your cheeks. Sans answers your pleas with gusto, ruts into you in a feral frenzy, and you _feel_ the searing heat of his breath against your cheek more than you hear him speak, catching snippets of the words snarled against your skin.  
  
  
  
“—gonna ruin ya for everyone else, mark ya over ‘n over ‘til everyone knows yer fuckin’ _mine_ —”  
  
  
“—pump yer belly full a’ little monsters, watch ya grow fat with my kids—”  
  
  
“—everyone ya meet is gonna smell me on ya, and yer never gon’ be sittin’ right again—”  
  
  
  
You couldn’t answer him if you wanted to: all you can manage is sob and writhe underneath him while he takes his fill—and in the moment, there is nowhere you would rather be. When his rhythm stutters and his litanies cut off on a broken curse, you barely have the time to acknowledge the hot cum filling you to the brim before you find yourself on your back, your legs drawn high and slung over Sans’s shoulders as he groans deeply into the crook of your neck, slivered syllables of what might have been your name scorching your sweat-slicked skin as he rocks against you with the same, delirious urgency.  
  
You have no clue how many times you come after that, just like you lose count of the times Sans comes inside you. All you know is that by the time he rolls off you, having emptied himself inside you for the umpteenth time, a part of you expects him to turn you around and keep on going. To say you’re sore would be an understatement—you feel like you just ran a triathlon and celebrated with a hike across the desert. The fulfilling throb in your battered cunt is echoed throughout your body, most apparent around the fucking necklace of bruises adorning your neck and chest—plus a handful of stray ones on your knees and elbows, courtesy of Sans deciding that you falling off the bed simply called for him to continue to fuck you on the floor.  
  
“At least you didn’t knot me,” you joke, your throat parched and raw from screaming. Sans gives you a blank stare from where he’s sprawled out on the bed, sockets at half-mast and eyelights hazy. In lieu of an answer, he reaches for the jug of ice water on the night stand—how it didn’t get overturned throughout your sexcapades is beyond you—and hands it to you without a word. The relief that floods your body as the cold water flows down your throat is damn near orgasmic, as you’re pretty sure your borderline-indecent moan is certain to attest. Once you’re done scarfing down two thirds of the contents, Sans takes the jug from you and gives it a contemplative once-over, before shrugging and pouring the remainder over himself. With the sound he makes, you bitterly conclude the two of you are tied for whore moans.  
  
“Really? You think this bed needs any more wet spots?” To call the path of destruction you’ve left in your wake a bed is a stretch. The blanket and most of the pillows had been chucked to the floor before you’d entered the scene, but you are thoroughly confounded as for how you’d managed to get the sheets into a celtic knot.  
  
“shuddup, ‘m tryin’ ta break a record here.” Sans rolls onto his side and reaches out as if to pull you into his arms—then promptly recalls that being being smushed up against a monster whose body temperature exceeds forty Celsius, let alone one who’s been railing you for an hour straight, would be the antithesis of aftercare.  
  
“The record for percentage of room covered in body fluids? ‘Cause I’m pretty sure you’ve got that covered.” You can’t help yourself, you have to steal a kiss—and you can feel the residual tension melting from Sans’s shoulders as he returns it, tongue flicking softly over your swollen lips, and one hand coming up to cup your cheek.  
  
“actually, ‘m pretty sure yer the one who got most of it covered.” He nods towards the aforementioned wet spots, the familiar shit-eating sneer creeping back into his smile. Then his gaze drops to your shoulder, and his expression falls. “aw, shit. gotta fix that, c'mere.”  
  
The bites he branded you with, while hilariously excessive by sheer virtue of their number, are no worse than the usual trophies of your play—but the prospect of going out in public looking slightly less like you had been mauled by a bear strikes you as a sound idea. His concern tugs at something alarmingly close to your heartstrings, and you scoot closer with a grateful smile, keen to feel the warm tingle of healing magic that had grown on you as something oh-so-special.  
  
That only makes it all the more jarring when, instead of that soothing warmth, that familiar magic scatters across your skin in scalding sparks. Your yelp is one of surprise more than of pain, but Sans still recoils like he’d just touched a hot stovetop.  
  
“shit! fuckin’ heat crap— keep forgettin’ ya humans are so fuckin’ fragile…” He gives you an apologetic grin, guilt clear as day in his eyelights.  
  
You’re far more concerned with the last part of his statement. Your scandalized gasp is one of your best ones yet as you lean in to flick him lightly on the forehead. “I just got pummeled by a metric fuckton of heat-fueled monster! You don’t get to call me fragile!”  
  
Sans snorts a laugh, catching your swatting hand so he can press a kiss to your knuckles. “ya make a salient point, kitten.”  
  
You hum your assertion, threading your fingers with his as you ease back into the surprisingly comfortable absence of pillows. Sans watches you with hazy eyelights, a delightfully fucked-out grin on his face, until his eyelights dart back to your shoulder. “did i burn ya badly?”  
  
You reach around with your free hand to poke at the reddened skin on your shoulder. It stings a bit, no worse than a light sunburn. You’ll certainly take it over looking like you had an auto-erotic mishap with beartraps. “It’s fine. I’ve burned myself worse in the shower.”  
  
You can tell it’s not the right answer by the furrow in his brow. “my bro’s got artificial green in his room. lemme get ya some—”  
  
You reach out to catch him by the wrist before he has the chance to bolt. “Red, for the love of dog, don’t steal from your brother’s stash to heal my sex wounds. We’ll never hear the end of it.”  
  
Sans’s sockets narrow as he mulls over your argument, until the threat of his brother’s bitching inevitably takes the upper hand, and he sinks back into the mattress with a grumble. You have to roll your eyes.  
  
“If it’ll make you feel better, go get me an ice pack.”   
  
Sans perks up at your words, hazed-out eyelights sharpening as he regards the bundled mess that was once his sheets, then scoots off the end that he’s lying on and pulls. You groan in dissent as part of the fabric is yanked out from under your ass, but your displeasure lasts no longer than it takes Sans to locate the wet patch where the ice water he’d doused himself with had splashed on the sheets, and press the still-cool fabric to your shoulder.  
  
“And they say chivalry is dead.” You grin up at Sans, and he grins back as he flips you a half-hearted finger. “Really though, is it over?” Sans raises an eyebrow, and you clarify, “Your heat.”

The dry chuckle that earns you is all the answer you need. “yeah, sure is. ya got ‘bout twenty minutes before it hits again.”  
  
You wish you could say you were surprised. “Why’d you bother healing me then, you dingus?”  
  
Another chuckle, but this time it’s laden with something else. A promise. “thought i’d clear the slate so i could mark ya all over again. guess i’ll have ta get creative.” His already deep voice dips lower as his eyelights rove your body, the red turned darker with the hunger from before. A thrill of arousal mixed with dread bounds up your spine, and your pulse quickens when it dawns on you just why Boss wouldn’t be back until morning.  
  
  
…  
  
  
You feel like you’re gonna have a _great_ time.

**Author's Note:**

> hi did you know that i love everyone who reads my stuff? there are two things that give my life meaning: calculus and writing filthy skeleton porn, and i'm all out of calculus, so tHANK yOU!!!!
> 
>   
> omitted from this fic is the part where Red, courtesy of senses gone haywire with heat, hears/smells reader at the front door, and makes a mad dash for his shorts bc call him what you will but he’s nOT having this conversation with his entire dick out


End file.
